Conventionally, a control system described in Patent Literature 1 is known which controls assist torque for assisting the steering force of the driver via an electrically-driven power steering device as an actuator. The electrically-driven power steering device includes an electric motor that generates assist torque, and the control system includes a thermistor that detects the temperature of the electric motor, and a vehicle speed sensor that detects vehicle speed.
The control system determines application voltage to the electric motor, based on the temperature of the electric motor, which is detected by the thermistor, and the vehicle speed detected by the vehicle speed sensor, thereby controlling the assist torque. First, the temperature of the electric motor is compared with a predetermined upper limit value, and when the temperature of the electric motor is lower than the upper limit value, the temperature of the electric motor is compared with a predetermined warning temperature lower than the upper limit value. Then, according to the vehicle speed, a table for a temperature range lower than the warning temperature is searched when the temperature of the electric motor is lower than the warning temperature, and a table for a temperature range not lower than the warning temperature is searched when the temperature of the electric motor is not lower than the predetermined warning temperature, whereby the application voltage is calculated.
On the other hand, when the temperature of the electric motor is not lower than the upper limit value, it is determined that the temperature of the electric motor has reached burning temperature, and hence the application voltage is set to a value of 0. This stops the electric motor from rotating thereafter, whereby overload operation of the electric motor is avoided, and the burnout of the electric motor is avoided.
Further, another control system as described in Patent Literature 2 is known which controls assist torque via an electrically-driven power steering device including an electric motor. This control system includes an ambient temperature sensor that detects the ambient temperature of the electric motor, a steering torque sensor that detects steering torque, a vehicle speed sensor that detects vehicle speed, and so forth.
The control system determines supply current to the electric motor, based on the steering torque detected by the steering torque sensor and the vehicle speed detected by the vehicle speed sensor, thereby controlling the assist torque. Further, to avoid overload operation of the electric motor, the temperature of the electric motor is estimated based on the square of the value of electric current flowing through the electric motor and the ambient temperature of the electric motor, and when the estimated temperature is higher than a reference value, the supply current to the electric motor is restricted. This makes it possible to avoid overload operation of the electric motor, and the burnout of the electric motor.
[Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. H07-112666
[Patent Literature 2] Japanese Laid-Open Patent Publication (Kokai) No. 2003-284375
In the control system of Patent Literature 1, the thermistor for detecting the temperature of the electric motor is required, which increases the manufacturing cost, and when the thermistor becomes faulty, there is a fear that the electric motor enters an overloaded state, and failure, such as the burnout of the electric motor, is brought about. Further, it is difficult in design to provide the thermistor for detecting the temperature at a rotating part of the electric motor, and hence after all, the temperature at a location having correlation to the coil temperature of the electric motor is detected. As a result, there is a fear of the temperature measurement being delayed to delay the detection of an overloaded state of the electric motor.
Further, in the control system of Patent Literature 2, in calculation of the estimated temperature, the square of the value of electric current flowing through the electric motor is used, variation in the estimated temperature due to a change in the value of electric current is increased. Therefore, in determination for avoiding overheat of the electric motor, it is difficult to properly set the reference value which is to be compared with the estimated temperature, which can bring about an error in the determination. In addition, the ambient temperature sensor is required, which increases the manufacturing cost, and when the ambient temperature sensor is faulty, it becomes impossible to properly estimate the temperature of the electric motor.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a control system which is capable of avoiding an overloaded state of an actuator without using sensors or the like.